The present invention was developed as a compaction vehicle having an engine driven pump and parallel hydraulic motors respectively driving a forward roll and rear wheel. Normally the parallel connection of the motors allows any differential rotation of the two wheel sets of a vehicle including such as may be due to changes in their effective diameters or loss of traction by one or the other wheel set. The prior art includes flow proportioners which enforce a preselected division of the flow to each wheel set. Some proportioners are such that they must be by-passed to allow limited slipping of the wheel sets such as for high speed travel. Such a by-pass in a compaction roller is shown, for example, in U.S. Pat. No. 3,889,759.
The object of the present invention is to provide an improved limited slip control where, for example, the forward wheel set is a vibrating steel compaction roll and the rear wheel set comprises differentially driven rubber tired wheels and where the tires may have cleats for maximum traction in one direction but not necessarily in the other direction.
A first problem is due to the normal requirement that the tired wheels propel the roller whereas the vibratory roll should not overspeed or underspeed. That is, for example, where the roll is compacting hot asphalt, if the induced roll rotation is too fast or slow relative to ground speed the asphalt is either pulled under the roll or pushed ahead of the roll. This results in an uneven finished surface. The roll, of course, is at the heavier end of the machine. The lighter end is supported by smooth tires which will not mark the asphalt. The second problem develops where the vibratory roller must operate up and down a grade such that additional weight is on the roll and less on the tires or vice versa.
A particular problem develops where the roller is to compact loose soil and the tires are cleated for the required traction which may not be the same in both directions because of the configuration of the cleats. Such a roller may also include a roll having compaction feet so that while the traction of the roll may be substantially more than that of the tires, slipping of the roll in particular will dig out the compacted soil and require adding fill and recompacting.
Another problem develops when the roller is fitted or operated with auxiliary equipment. This may include a forward blade for earth-moving or second compaction roll which is drawn by the roller.
As another matter, the two motors driving the roll and the wheels may be and generally are of entirely different hydraulic characteristics. For example, such a roller may have the following specifications:
______________________________________ Gross weight: 14,000 lbs (5900 KG) Wheelbase: 104 inches (2565 mm) Drum diameter: 48 inches (1219 mm) Drum width: 60 inches (1819 mm) Cleat diameter: 55-1/2 inches (1403 mm) Roll motor: 20.65 cu. in. (524.51 mm.sup.3) displ. Roll drive reduction: 6.8-1 Tires 17 .times. 24 Diameter: 58 inches (1473 mm) Wheel motor: 5.43 cu. in. (137.92 mm.sup.3) displ. Wheel drive reduction: 24.12-1 ______________________________________
Separately multiplying pump displacement and gear reduction for the roll and for the wheels as shown above and dividing each product respectively by the radius of the roll and of the wheel will provide a ratio near unity (one). The object of the invention as made was to provide flow and pressure responsive control of the input to each of the motors so that when either wheel set, that is the roll or a wheel, loses its traction, spinning is minimized and additional tractive effort to the other wheel set is immediately effected having due regard for whatever such ratio may be.
Another object was to make the transfer of power from the spinning wheel set separately adjustable so that the different requirements depending upon both the direction of travel and the direction of travel respecting a gradient may be provided as required.
Another object has been to allow the use of flow restrictors so that rate of flow may be sensed but without materially reducing the efficiency of the hydraulic transmission of power from the engine to the wheel sets at any speed.
Another object has been to provide a slip control which does not require any manual control or operator attention.